All fluorescent lamps use a phosphor blend or mix which is excited by the mercury arc stream discharge ultraviolet radiation to produce light. The typical phosphor blend comprises a variety of types of phosphors to produce the desired spectral radiation light output.
Two types of phosphor blends are currently in use. The more common one, which has been in use for a long period of time, uses a mix of so-called wide band phosphors. Each of these phosphors can produce light over a relatively wide wavelength band, for example about 60-160 nanometers, of various parts of the visible spectrum. The quantity of each of the phosphors in the blend is selected so that the desired color temperature of light is produced with desired light output, usually measured in lumens. While phosphor mixes using such wide band phosphors are effective, they generally are not as efficient in light output as narrow band phosphors.
The phosphors of a blend are also selected to produce a desired color rendering index (CRI). The higher the CRI index approaches to 100, the closer will be the color of the light to a reference color. This reference is a heated "black body" up to 5000K and phases of natural daylight above 5000K. A blend using wide band phosphors can have a relatively high color rendering index, even as high as 91-93, but with relatively low lumen output.
Another type of phosphor blend uses several of the less common but more efficient rare earth phosphors. These rare earth phosphors are more efficient than the wide band phosphors in that a blend of such rare earth phosphors can have a relatively high lumen output. Typically, they are used in a blend of three such rare earth phosphors to produce the visible light output. While the visible light output and the lumens per watt efficacy is high, the color rendering index of a typical blend of three rare earth phosphors is relatively low, usually in the range from about 73-80.
It is also sometimes desireable to produce a fluorescent lamp whose visible light and ultraviolet energy output spectrum approximates that of natural daylight for a given correlated color temperature. Such a lamp is disclosed in U.S. Pat. No. 3,670,193, Thorington, et al., which is assigned to the same assignee as this application. The phosphor blend of that lamp relies upon wide band phosphors for producing the visible light output. Another phosphor is added to the blend to produce energy in the ultraviolet range so that the total radiated spectrum output of the lamp in both the visible and ultraviolet range simulates natural daylight at a given correlated color temperature.
In the copending application, Ser. No. 108,895, filed Oct. 15, 1987 entitled PHOSPHOR BLEND FOR BROAD SPECTRUM FLUORESCENT LAMP, abandoned in favor of continuation application Ser. No. 346,317, filed May 1, 1989, now U.S Pat. No. 4,891,550 granted Jan. 2, 1990, which is also assigned to the same assignee, a phosphor blend for fluorescent lamps is disclosed which uses a combination of wide band phosphors and narrow band rare earth phosphors. By suitably selecting the combination of narrow band rare earth and wide band phosphors, the lumen output and color rendering index can be controlled. The general principles disclosed are that the phosphor blend components are selected so that as the lumen output of the lamp increases, the color rendering index decreases, and vice versa. Lamps according to the teachings of that application also can have a suitable ultraviolet energy emitting phosphor in the blend so that the overall spectrum of the lamp can approach that of natural daylight as in the aforesaid Thorington et al. patent.
The blends using three of the more efficient rare earth phosphors leave gaps in their spectrum of visible light output. These gaps account for a lower color rendering index for usually higher lumen output compared to blends using the less efficient wide band phosphors. That is, since there are gaps in the overall color spectrum, a high CRI cannot be achieved.